Resolve the native Windows std directory when HOME is absent

[LunaStev]

Confirmed failure after native linking succeeds

PR #486 head 6fb6ed7 resolves the missing xml2s.lib dependency far enough to build the native Windows ARM64 release compiler, pass Clippy and run all three native unit tests. The next target-object smoke step fails with:

HOME env not set; cannot locate std at ~/.wave/lib/wave/std

Native build job

The native ARM64 cases job also has ten cases reporting this exact error. Its other 113 access violations are tracked in #493 as a separate failure and must not be attributed to HOME without evidence.

Cause

The PowerShell workflow installs std under $HOME, which is a PowerShell automatic variable. Rust's std::env::var("HOME") reads an environment variable, which need not be set on native Windows even when $HOME and USERPROFILE are available.

front/parser/src/import.rs::std_root_dir and src/std.rs::resolve_std_install_dir currently require the HOME environment variable. Installation and discovery must agree on the same Windows user directory.

Bounded contribution

Implement and document a consistent platform-aware user-directory policy for std installation and lookup. Keep an explicitly supplied HOME working; use the appropriate Windows home-directory fallback when it is absent. Audit the native build/cases/release setup so they install to the path the compiler will actually use. Avoid a CI-only workaround that leaves native users unable to import std.

Acceptance

• A native Windows process with HOME absent and a valid Windows user profile resolves the same .wave/lib/wave/std directory for installation and imports.
• Existing HOME-based Unix and explicit override behavior is preserved.
• Add focused tests for precedence, missing values and consistent installation/import paths; do not mutate the global environment concurrently in tests.
• The native ARM64 target-object smoke reaches object generation and the ten HOME-related case failures are eliminated.
• Do not claim this fixes the separate access violations or all native cases.

Start in front/parser/src/import.rs, src/std.rs, .github/workflows/rust.yml, .github/workflows/cases.yml and .github/workflows/release.yml. Related: #482, #486, #292. The directory-resolution change is small enough for a first contribution; native CI supplies the platform verification.